1871.] On the Telescopic Appearance of Encke's Comet. 87 



b. Comparison of B and C. 



Duration of Duration of 



Periods. ascent (B). Periods. descent (C). 



I. 1833-92 to 1836-98 3-06 years 1836*98 to 1843-75 677 years. 



II. 1843-75 to 1847*87 4-12 „ 1847*87 to 1856-31 8-44 „ 



III. 1856-31 to 1859*69 3'38 „ 1859-69 to 1867*12 7*43 „ 



"D f* ^ 



itatio -. Difference from mean. 



1 2-212 ] + 0-061. 



II 2-044 \ Mean 2*151 -0*107. 



Ill 2-198 J +0*047. 



The agreement of these ratios with each other, and the small differences 

 from the mean of the single ratios, justify us in the mean time, until a 

 greater number of periods are before us, to state the connexion between 

 the two branches of the periodic curve from one minimum to another in 

 the following more precise terms : — 



2/T be the time of duration of sun-spot increase from a minimum to a 

 maximum, then 2*15 X T (with a probable error of less than +0-05) will 

 be the duration of the sun-spot decrease until the next minimum. 



This law, together with the fact which we have previously established, 

 that a longer period shows generally a depressed curve, while a shorter is 

 characterized by great peaks, points strongly to the conclusion that the 

 energy of the ultimate causes of sun-spot production, vjhether these causes 

 be intrasolar or extrasolar, is for every period constant. 



IV. " Note on the Telescopic Appearance of Encke's Comet." By 

 William Huggins, D.C.L., LL.D., F.R.S. Received De- 

 cember 16, 1871. 



The first three figures which accompany this note represent the comet 

 on evenings on which its appearance was described in a note on the 

 spectrum of the comet which I had the honour to present to the Royal 

 Society*, A continuance of bad weather prevented me from making later 

 observations of the comet, with the exception of one evening, December 5, 

 when figure 4 was obtained under unfavourable circumstances. 



Fig. 1. November 7, 7.30 p.m. — From Oct. 17, when the comet con- 

 sisted of a nearly round nebulosity without condensation in any part, 

 to Nov. 7 no observations could be obtained. At the latter date, the re- 

 markable fan-form which distinguishes this appearance of the comet was 

 already distinctly presented. The faint light by which the comet was sur- 

 rounded terminated on the side from the sun, that from which the tail is 

 usually projected, in a straight boundary at right angles to the longer axis 

 of the comet. At the opposite side, that towards the sun, the faint nebu- 

 losity expanded and became fainter until it could be no longer traced. The 



* Supra, p. 45. 



